DESCRIPTION: (Applicant's Abstract) The long term goal of this project is to develop methods for induction of peripheral T cell tolerance, that could improve prevention of graft rejection and graft-versus-host disease (GVHD) after human marrow transplantation. The applicant has established that activation driven-cell death is induced selectively in antigen-activated, cycling CD4+ or CD8+ T cells by soluble anti-CD3 epsilon F(ab')2 antibody fragments. These results suggested that anti-CD3 antibodies can induce immunosuppression by activation-dependent T cell death. The central aim of this application is to determine whether treatment with anti-CD3 antibodies achieves tolerance in MHC-incompatible murine model of GVHD by specific deletion of donor T cells that recognize recipient alloantigens. Specific aims are: 1) Determine whether in vivo treatment with anti-CD3 F(ab')2 prevents GVHD by deletion of allospecific T cells in mice. The applicant has chosen to study GVHD mediated by TCR-transgenic 2C cells because: A) the CD8+ TCR-transgenic 2C cells can cause GVHD in Ld+ mice, manifested by the depletion of recipient double positive thymocytes, and peripheral B and T cells; b) 2C cells can be visualized by staining for a TCR clonotypic determinant recognized by mAb 1B2, allowing him to determine the 2C cell fate. 2) Effect of TCR affinity for peptide:MHC on T cell fate. T cells with low affinity TCR may persist longer in an allogenic recipient and cause sustained GVHD more likely than T cells with high affinity TCR. He will test the effect of anti-CD3-treatment on the fate of CD8+ T cells from the 2C mouse transplanted into recipients that express alloantigens of various strengths: Ld, Kbm3, and Kmb11. 3) Effect of memory on T cell fate. Memory T cells have high avidity for antigen and antigen presenting cells than naive T cells and develop a more intense proliferative response to antigen. Antigen-primed T cells may cause more severe GVHD, but they should be very susceptible to the effects of anti-CD3 (F(ab')2. 4) Effect of help on T cell fate. CD+ T cells can provide help to CD8+ T cells by releasing cytokine, including IL-2, IL-12 and gamma-IFN, that promote cell replication and effector function, and by their surface expression of accessory molecules. He will test the hypotheses that CD4+ T cells enhance and sustain the activity of CD8+ cells, and that treatment with anti-CD3 F(ab')2 can abrogate CD4+ T cell help.